Method of testing regenerative braking force in electric vehicle

ABSTRACT

In an electric vehicle having front wheels braked by regenerative braking and hydraulic braking and rear wheels hydraulically braked, it is possible to check the magnitude of the regenerative braking force easily and accurately, by using a two-axle chassis dynamometer. Assuming that the front wheel braking force F 2  &#39; detected by the chassis dynamometer is the total braking force C 2  &#39; of the front and rear wheels, the total braking force C2&#39; is broken down into component forces according to braking force distribution ratio data previously stored in a memory, to calculate a reference regenerative braking force A 2  &#39;D 2  &#39;. Based on the actually detected pedal depressing force A 2  and the distribution ratio data, an imaginary pedal depressing force A 2  &#39; is calculated that is required to produce a total braking force of the driven wheels and follower wheels equal to the braking force F 2  &#39;, generated by the driven wheels in the actual pedal depressing force A 2 . Based on the imaginary pedal depressing force A 2  &#39; and the distribution ratio data, the imaginary regenerative braking force A 2  &#39;D 2  &#39; that is produced by the driven wheels in the imaginary pedal depressing force A 2  &#39;, is calculated and then compared with the reference regenerative braking force A 2  &#39;D 2  &#39;.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of testing the regenerativebraking force of the driven wheels of an electric vehicle by using atwo-axle chassis dynamometer in the vehicle in which the driven wheelscan be hydraulically braked and regeneratively braked.

2. Description of the Prior Art

In an electric vehicle which travels by driving the driven wheels withan electric motor, a priority is given to regenerative braking to brakethe driven wheels and thereby to effectively recover energy. When asufficient braking force is not obtained with the regenerative brakingalone, the hydraulic braking is also actuated to secure a sufficienttotal braking force.

The braking force of an electric vehicle produced in a factory is testedusing a chassis dynamometer to judge whether the regenerative brakingforce is produced normally or not. Since the distribution characteristicof the regenerative and hydraulic braking forces with respect to thebrake pedal depressing force is stored in advance, the determination ofwhether the regenerative braking force is produced normally is made bycalculating the regenerative braking force based on the brake pedaldepressing force and the braking force distribution characteristic,calculating the value of the regenerative braking force based on thebraking force detected by the chassis dynamometer and the braking forcedistribution characteristic, and comparing the two values.

There may be a situation where a four-axle chassis dynamometer that canmeasure the braking force of the driven wheels and the braking force ofthe follower wheels at the same time is not available for the test onthe braking force of an electric vehicle, and therefore a two-axlechassis dynamometer must be used. In such a case, if the braking forcemeasured by the two-axle chassis dynamometer, is used as is, an errormay be included in the results of test because the braking forcemeasured by the two-axle chassis dynamometer includes only the brakingforce of the driven wheels and does not include the braking force of thefollower wheels. Hence, the use of the two-axle chassis dynamometerrequires a troublesome correction processing.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of thecircumstances described above and has an object to enable easy andaccurate test on the regenerative braking force of an electric vehicleby using a two-axle chassis dynamometer.

According to the present invention, the braking force of the drivenwheels is detected by the two-axle chassis dynamometer; and assumingthat the detected braking force is a total braking force of the drivenwheels and follower wheels, the reference regenerative braking forceproduced by the driven wheels is calculated on the basis of the dataobtained by determining the braking force distribution to the drivingand follower wheels in advance, as a function of force applied to thebrake pedal. Based on the actual pedal depressing force and the data, animaginary pedal depressing force is calculated that is required toproduce a total braking force of the driven wheels and follower wheelsequal to the braking force generated by the driven wheels in the actualpedal depressing force. Based on the imaginary pedal depressing forceand the data, an imaginary regenerative braking force that is producedby the driven wheels in the imaginary pedal depressing force iscalculated and then compared with the reference regenerative brakingforce. In this way the regenerative braking force test is conducted.

When an electric vehicle with an ABS device is mounted on the two-axlechassis dynamometer, the ABS device is activated because the followerwheels do not rotate and are locked, limiting the regenerative brakingof the driven wheels and rendering the regenerative braking force testimpossible. During the test using the two-axle chassis dynamometer,however, the regenerative braking of the driven wheels can be performedeven when the ABS device is actuated, thus allowing the regenerativebraking force test to be conducted without trouble.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall configuration of an electric vehicle having abraking system in an embodiment of the present invention.

FIG. 2 is a graph showing a distribution ratio between the hydraulicbraking force and the regenerative braking force of the front and rearwheels.

FIG. 3 is a graph showing the input/output characteristic of the offsetvalve.

FIGS. 4A and 4B are explanatory diagrams showing the chassisdynamometers.

FIG. 5 is a flow chart showing the procedure of the regenerative brakingforce test.

FIG. 6 is a flow chart showing the procedure of calculating the brakingforce distribution ratio for the regenerative braking force test.

FIG. 7 is a graph showing the first pattern of the braking forcedistribution ratio.

FIG. 8 is a graph showing the second pattern of the braking forcedistribution ratio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The mode for carrying out the present invention will be described inconjunction with an embodiment shown in the accompanying drawings.

First, the overall configuration of the braking system of an electricvehicle will be described referring to FIG. 1. The electric vehicle is afour-wheel motor vehicle having a pair of front wheels Wf, Wf as drivenwheels and a pair of rear wheels Wr, Wr as follower wheels, with thefront wheels Wf, Wf connected through a transmission 3 and adifferential gear 4 to an electric motor 2, which has a battery 1 as anenergy source. Installed between the battery 1 and the electric motor 2is a PDU (power drive unit) 5 which controls the driving of the electricmotor 2 by the battery 1 and also controls the charging of the battery 1with the electricity generated by the electric motor 2 during theregenerative braking. The power drive unit 5 is connected to the motor'selectronic control unit (motor ECU) 6, which in turn is connected to abrake ECU 7.

A negative pressure booster 10, which amplifies the force applied to abrake pedal 8 and transmits the amplified force to a master cylinder 9,is connected to a negative pressure tank 11, whose pressure is reducedby a negative pressure pump 13 driven by a negative pressure pump drivemotor 12. A rear output port 9r of the master cylinder 9 is connected tobrake cylinders 16r, 16r of the left and right rear wheels Wr, Wrthrough a modulator 15 controlled by an ABS (antilock braking system)ECU 14. A front output port 9f of the master cylinder 9 is connected tobrake cylinders 16f, 16f of the left and right front wheels Wf, Wfthrough an offset valve 17 comprising a solenoid valve and the modulator15.

Wheel-speed sensors 18f, 18f; 18r, 18r provided to the front wheels Wf,Wf and the rear wheels Wr, Wr are connected to the ABS ECU 14. The ABSECU 14 actuates the modulator 15 to reduce the brake pressuretransmitted to the brake cylinders 16f, 16f; 16r, 16r when the frontwheels Wf, Wf and/or rear wheels Wr, Wr tend to be locked. The brake ECU7 connected to a hydraulic pressure sensor 19, which detects the outputoil pressure Pm of the front output port 9f of the master cylinder 9(i.e., the depressing force applied to the brake pedal 8), controls theregenerative braking force of the electric motor 2 through the motor ECU6 and the PDU 5.

FIG. 2 shows a braking force distribution characteristic of the frontwheels Wf, Wf and the rear wheels Wr, Wr with respect to the depressingforce applied to the brake pedal 8. This data is stored previously inthe brake ECU 7. The front wheels Wf, Wf as the driven wheels, arebraked by both the hydraulic and regenerative brakings. The regenerativebraking is executed first until the output oil pressure Pm of the mastercylinder 9 reaches the offset oil pressure Pmo. When the regenerativebraking force reaches the regeneration limit value R₀, the hydraulicbraking is then initiated while maintaining the regeneration limit valueR₀. As a result, the braking force of the front wheels Wf, Wf increaseslinearly with the output oil pressure Pm. On the other hand, the rearwheels Wr, Wr as the follower wheels, are braked only by hydraulicpressure and the braking force of the rear wheels Wr, Wr increaseslinearly with increase in the output oil pressure Pm. By performing theregenerative braking in preference to the hydraulic braking when brakingthe front wheels Wf, Wf as described above, the kinetic energy of thevehicle body that would be wasted as heat energy during hydraulicbraking, can be recovered by the regenerative braking as electricenergy, that can be used for charging the battery 1, thus extending thetravel range for one charging.

FIG. 3 shows the characteristic of the offset valve 17, in which whilethe output oil pressure Pm of the master cylinder 9 increases from zeroto the offset oil pressure Pmo, the output oil pressure Pv of the offsetvalve 17 is kept at zero, and when the output oil pressure Pm of themaster cylinder 9 further increases, the output oil pressure Pv of theoffset valve 17, increases linearly from zero. Therefore, when the brakepedal 8 is depressed, the hydraulic braking force on the rear wheels Wr,Wr that are not connected with the offset valve 17, increases inproportion to the force applied to the brake pedal 8, whereas nohydraulic braking of the front wheels Wf, Wf that are connected with theoffset valve 17, is performed until the brake pedal depressing forcereaches a predetermined value (i.e., until the output oil pressure Pm ofthe master cylinder 9 reaches the offset oil pressure Pmo). During thisperiod the regenerative braking force increases with increase in thebrake pedal depressing force, and when the output oil pressure Pm of themaster cylinder 9 reaches the offset oil pressure Pmo and theregenerative braking force reaches the regeneration limit value R₀, theoutput oil pressure Pv of the offset valve 17 rises, starting thehydraulic braking on the front wheels. The value of offset oil pressurePmo can be controlled arbitrarily by a command from the brake ECU 7.

The output oil pressure Pm of the master cylinder 9 detected by thehydraulic pressure sensor 19, is inputted into the brake ECU 7, which,while the output oil pressure Pm of the master cylinder 9 increases fromzero to the offset oil pressure Pmo, linearly increases the regenerativebraking force of the electric motor 1 to the regeneration limit valueR₀, and after the output oil pressure Pm has exceeded the offset oilpressure Pmo, maintains the regenerative braking force at theregeneration limit value R₀.

The test for checking whether an electric vehicle produced in a factory,can produce an appropriate regenerative braking force, includes thesteps of simulating the actual traveling condition of the vehicle usinga chassis dynamometer. There are two types of chassis dynamometer, is afour-axle chassis dynamometer that supports the front wheels Wf, Wf andthe rear wheels Wr, Wr with two-axle rollers respectively (see FIG. 4A)and a two-axle chassis dynamometer that supports only the front wheelsWf, Wf, the drive wheels, with two-axle rollers (see FIG. 4B). Thefour-axle chassis dynamometer D₄ is able to measure both the brakingforce of the front wheels Wf, Wf and the braking force of the rearwheels Wr, Wr but is not widely used because it is expensive. Thetwo-axle chassis dynamometer D₂, which is comparatively inexpensive andwidely used, can only measure the braking force of the front wheels Wf,Wf. The use of the four-axle chassis dynamometer D₄ enables theregenerative braking force test to be performed without causing anyproblems, by simulating the actual traveling condition of the vehicle.With the two-axle chassis dynamometer D₂, however, the regenerativebraking force test cannot be carried out accurately. This will beexplained below.

The test on the regenerative braking force is performed in a conditionin which an external diagnosing device T (see FIG. 1) connected to thechassis dynamometer is connected to the brake ECU 7, by simulating theactual traveling conditions with the electric vehicle mounted on thechassis dynamometer, and by depressing the brake pedal 8 by the driver.

When the driver depresses the brake pedal 8, the total braking force ofthe front wheels Wf, Wf and rear wheels Wr, Wr in the case of thefour-axle chassis dynamometer D₄ or the braking force of only the frontwheels Wf, Wf in the case of the two-axle chassis dynamometer D₂, isdetected by the chassis dynamometer D₄ or D₂ and outputted to thediagnosing device T. The diagnosing device T compares the regenerativebraking force calculated from the braking force detected by the chassisdynamometer D₄ or D₂, with the regenerative braking force output fromthe brake ECU 7 to determine if normal braking is performed.

When the four-axle chassis dynamometer D₄ is used, the above judgmentcan be made without any additional correction. In more concrete terms,if the braking system works normally, when the total braking force ofthe front wheels Wf, Wf and rear wheels Wr, Wr is, for example, F₁ ' inFIG. 7, the regenerative braking force of the front wheels Wf, Wf mustbe A₁ 'B₁ ', the hydraulic braking force of the front wheels Wf, Wf mustbe zero, and the hydraulic braking force of the rear wheels Wr, Wr mustbe B₁ 'C₁ '. Hence, the regenerative braking force of the front wheelsWf, Wf (for example, A₁ 'B₁ ') is calculated from the total brakingforce (for instance, F₁ ') on the basis of the braking forcedistribution ratio previously stored in the brake ECU 7. If an errorbetween this calculated regenerative braking force and a command valueof the regenerative braking force output from the brake ECU 7, issmaller than a predetermined value, it is determined that a normalbraking force is produced.

When the two-axle chassis dynamometer D₂ is used, only the braking forceof the front wheels Wf, Wf is considered while the braking force of therear wheels Wr, Wr is not considered. Hence, in order to make thebraking force detected by the two-axle chassis dynamometer D₂ equal tothe braking force detected by the four-axle chassis dynamometer D₄, agreater brake pedal depressing force is necessary. For example, in FIG.7, when the output oil pressure Pm of the master cylinder 9 is A₁ ', thetotal braking force of the front wheels Wf, Wf and rear wheels Wr, Wr isF₁ '. To produce F₁ ' with only the braking force of the front wheelsWf, Wf detected by the two-axle chassis dynamometer D₂, the brake pedaldepressing force must be increased to A₁. The brake ECU 7 incorrectlydetermines that the total braking force of the front wheels Wf, Wf andrear wheels Wr, Wr is F₁ corresponding to the brake pedal depressingforce A₁, and calculates the regenerative braking force corresponding tothe braking force F₁ and obtains A₁ B₁. At this time the command valueof the regenerative braking force output from the brake ECU 7 is A₁ B₁that corresponds to the output oil pressure A₁ of the master cylinder 9,so that the regenerative braking force A₁ 'B₁ ' calculated from theoutput of the two-axle chassis dynamometer D₂ does not agree with thecommand value A₁ B₁ of the regenerative braking force output from thebrake ECU 7, rendering a correct judgment impossible.

This invention is intended to perform a test on the regenerative brakingforce by using an inexpensive two-axle chassis dynamometer D₂, and forthis purpose, the brake ECU 7 changes the braking force test mode.

Now, the operation of the preferred embodiment will be describedreferring to the flow chart.

The flow chart of FIG. 5 shows the overall processing flow of theregenerative braking force test using the two-axle chassis dynamometerD₂. If at Step S1 it is determined that the regenerative braking forcetest using the two-axle chassis dynamometer D₂ is to be performed, theprocedure moves to Step S2 where a command from the diagnosing device Tchanges the mode of the brake ECU 7 to the regenerative braking forcetest mode, and at Step S3 an alarm lamp is turned on to indicate thatthe mode has changed from the normal mode to the test mode.

When it is necessary, in the subsequent Step S4, to change the brakingforce distribution ratio for the regenerative braking force test becauseof a change of a tire or brake pad of the electric vehicle, thediagnosing device T at Step S5 modifies the braking force distributionratio for the test stored in the brake ECU 7. Then, at Step S6 theregenerative braking force test of the front wheels Wf, Wf is performed.After the regenerative braking force test is finished at Step S7, thediagnosing device T changes the mode of the brake ECU 7 from the testmode back to the normal mode at Step S8, turning off the alarm lamp atStep S9 and shifting to the normal mode control at Step S10.

FIG. 6 is another flow chart showing processing performed by the brakeECU 7 when conducting a regenerative braking force test. When at StepS11 the output oil pressure Pm of the master cylinder 9 is zero becausethe brake pedal 8 is not depressed, or when, even if the brake pedal 8is depressed at Step S11, the regenerative braking is limited orinhibited at Step S12, the regenerative braking force R is set at avalue a smaller than the normal value, R₀, or at zero.

The condition for limiting the regenerative braking of an actuallytraveling electric vehicle is when the ABS system is operating or hasfailed, for example. The condition for inhibiting the regenerativebraking is when the vehicle speed is very slow, when the ignitionvoltage is below a minimum voltage, or when the brake system has failed,for example. During the test using the two-axle chassis dynamometer D₂,however, the rear wheels Wr, Wr are at rest and consequently determinedto be locked, so that the ABS system operates. This causes theregenerative braking force R to be limited to a value a at all times inStep S18, rendering the regenerative braking force test impossible.Hence, during the test using the two-axle chassis dynamometer D₂, thecondition that "the ABS system is operating or failed" is removed fromthe conditions for limiting the regenerative braking in Step S12. Thisprevents the regenerative braking force R from being limited at StepS18, allowing the braking force test to be performed without anyproblem.

When at Step S11 the output oil pressure Pm of the master cylinder 9 isnot zero and at Step S12 the regenerative braking is not limited norinhibited, the corrected regenerative braking force R (hereinafterreferred to as an imaginary regenerative force R) is calculated in StepsS13 to S17. This calculation process will be described by referring toFIGS. 7 and 8. First, at Step S13 an imaginary output oil pressure A₁ ',A₂ ' that corresponds to the actual output oil pressure Pm of the mastercylinder 9 detected by the hydraulic pressure sensor 19 (i.e., actualoutput oil pressure A₁, A₂ in FIGS. 7 and 8) is calculated. In otherwords, when the actual output oil pressure of the master cylinder 9 isA₁ or A₂, the total braking force of the front wheels Wf, Wf and rearwheels Wr, Wr is theoretically F₁ or F₂. Because the braking forcedetected by the two-axle chassis dynamometer D₂ is F₁ ' or F₂ ' which isthe braking force of the front wheels Wf, Wf, an imaginary output oilpressure A₁ ', A₂ ' is calculated when assuming that the braking forceF₁ ', F₂ ' is the total braking force of the front wheels Wf, Wf andrear wheels Wr, Wr.

At the next Step S14, the regenerative braking force of the front wheelsWf, Wf (A₁ 'B₁ ' or A₂ 'B₂ ') is calculated from the total braking forceF₁ ', F₂ ' of the front wheels Wf, Wf and rear wheels Wr, Wrcorresponding to the imaginary output oil pressure A₁ ', A₂ ', based onthe braking force distribution ratio. At this time, the calculatedregenerative braking force A₁ 'B₁ ' or A₂ 'B₂ ' is compared with theregeneration limit value R₀ in Step S15. If, as shown in the firstpattern of FIG. 7, the regenerative braking force A₁ 'B₁ ' is equal toor lower than the regeneration limit value R₀, the regenerative brakingforce A₁ 'B₁ ' is taken to be the final imaginary regenerative brakingforce R in Step S16. On the other hand, if, as shown in the secondpattern of FIG. 8, the regenerative braking force A₂ 'B₂ ' exceeds theregeneration limit value R₀ (A₂ 'D₂ '), the regeneration limit value R₀at Step S14 is taken to be the final imaginary regenerative brakingforce R.

Then the brake ECU 7 calculates at Step S19 the control value from R×K₁(where K₁ is a proportional constant) to eliminate the hydraulic brakingforce corresponding to the imaginary regenerative braking force R fromthe hydraulic braking force of the front wheels Wf, Wf, and sends thecalculated control value to the offset valve 17. At Step S20, the brakeECU 7 calculates the control value based on R×K₂ (where K₂ is aproportional constant) to cause the electric motor 2 to generate aregenerative braking force equal to the imaginary regenerative brakingforce R and then outputs the control value to the electric motor 2.

The diagnosing device T assumes that the braking force of the frontwheels Wf, Wf detected by the two-axle chassis dynamometer D₂ is thetotal braking force F₁ ', F₂ ' of the front wheels Wf, Wf and rearwheels Wr, Wr of FIGS. 7 and 8, and calculates the regenerative brakingforce A₁ 'B₁ ', A₂ 'D₂ ' of the front wheels Wf, Wf (hereinafterreferred to as a reference regenerative braking force) on the basis ofthe braking force distribution ratio data.

Then, the diagnosing device T compares the reference regenerativebraking force with the imaginary regenerative braking force calculatedat the Steps S16, S17 and S18 shown in the flow chart of FIG. 6. If thedifference between reference and the imaginary regenerative brakingforces is smaller than a predetermined value, the diagnosing device Tdetermines that the electric motor 2 is producing a normal regenerativebraking force and, if the difference exceeds the predetermined value, itdetermines that there is something abnormal. Thus, the regenerativebraking force test can be performed easily and precisely by using thetwo-axle chassis dynamometer D₂ in the same way as when the four-axleschassis dynamometer D₄ is used.

Although the above embodiment concerns an electric vehicle that has thefront wheels Wf, Wf as driven wheels and the rear wheels Wr, Wr asfollower wheels, this invention is also applicable to an electricvehicle with the front wheels Wf, Wf being follower wheels and the rearwheels Wr, Wr being driven wheels. Further, while this embodimentdetects the force applied to the brake pedal 8 indirectly as the outputoil pressure Pm of the master cylinder 9, the force may be directlydetected by a pedal depressing force sensor.

According to the present invention, when the regenerative braking forcetest on the driven wheels is performed by using a two-axle chassisdynamometer, the imaginary pedal depressing force to produce the totalbraking force is calculated, assuming the detected braking force of thedriven wheels is a total braking force of the driven wheels and followerwheels. Based on this imaginary pedal depressing force, a determinationis made of whether the regenerative braking force of the driven wheelsis appropriate or not. This method allows the regenerative braking forcetest using the two-axle chassis dynamometer, to be conducted in the sameway as when the four-axle chassis dynamometer is used.

Further, according to the present invention, even when a vehicle to besubjected to the regenerative braking force test using the two-axlechassis dynamometer incorporates an ABS system, the test can beperformed without any trouble.

One embodiment of this invention has been described. It should be notedthat various design modifications may be made without departing from thespirit of this invention.

We claim:
 1. In an electric vehicle in which the braking force to drivenwheels is regenerative braking and hydraulic braking and to followerwheels is hydraulic braking, wherein the braking force distribution tothe driven and follower wheels is predetermined as a function of theforce of depressing a braking pedal and, braking is performed based ondistribution data; a method of testing the regenerative braking forceproduced by the driven wheels based on the depressing force of the brakepedal and the braking force of the driven wheels detected by a two-axlechassis dynamometer, comprising the steps of:detecting the braking forceof the driven wheels using the two-axle chassis dynamometer; calculatinga reference regenerative braking force produced by the driven wheelsaccording to the distribution data, assuming that the detected brakingforce is a total braking force of the driven wheels and the followerwheels; detecting an actual depressing force applied to the brake pedal;calculating, on the basis of the actually detected pedal depressingforce and the distribution data, an imaginary pedal depressing forcerequired to produce a total braking force of the driven wheels andfollower wheels equal to the braking force produced by the driven wheelsin the actually detected pedal depressing force; calculating, on thebasis of the imaginary pedal depressing force and the distribution data,an imaginary regenerative braking force produced by the driven wheels;and comparing the reference regenerative braking force and the imaginaryregenerative braking force.
 2. A regenerative braking force test methodin an electric vehicle according to claim 1, wherein the electricvehicle includes an ABS system, for inhibiting the regenerative brakingof the driven wheels when the follower wheels are locked, and allowingthe regenerative braking of the driven wheels even when the followerwheels are locked, during the test using the two-axle chassisdynamometer.